Neutralizing high-frequency pushpull amplifiers



May 23, 1939. w. BUSCH BECK ET AL 2,159,731

NEUTRALIZING HIGH-FREQUENCY.PUSH-PULL AMPLIFIERS Filed oct. s, 1936 2Sheets-Sheet 1 cur/160E INVENTORS W. BUSC H BECK BY H. FROST I ATTORNEYMay 23, 1939. w; B USCHBECK ET AL 2.15%131 NEUTRALIZING HIGH-FREQUENCYPUSH PULL AMPLIFIERS Filed Oct. 8, 1936 2 Sheets-Sheet 2 INVENTOR I w.BUSCH BECK BY H. PROST A QRNEY Patented May 23, 1939 PATENT OFFICENEUTRALIZING HIGH-FREQUENCY PUSH- PULL AMPLIFIERS Werner Buschbeck andHans Prost, Berlin, Germany, assignors to Telefunken Gesellschaft fiirDrahtlose Telegraphie m. b. H., Berlin, Germany, a corporation ofGermany Application. October 8, 1936, Serial No. 104,634 In GermanySeptember 13, 1935 3 Claims. (Cl. 179-471) the neutralization of highfrequency amplifiers can be rendered independent of frequency in thatnot only the gridanode capacity be neutralized, but also thecathode-grid capacity and the cathode-anode capacity. According toapplicant's proposal, this neutralization circuit is improved to" obtaina still better independence of frequency, in that not only thecapacities of the tube, but also the inductances of the electrodelead-ins are reproduced in the neutralization branches. 1

For a better understanding of the invention, reference is made to theaccompanying drawings, 151 in which:

Figs. 1,2 and are circuit diagrams of a pushfl p p ull amplifierneutralized in accordance with our 3 invention, v

,d Fig. 3 is a portion of the bridge circuit of Fig. 2 arranged toshowcertain voltage relations,

I i Fig. 4 is aportion of the bridge circuit of Fig. 2

l l l It is known from German Patent 532,515 that arranged to show othervoltage relations between A certain points and Fig. 6 isv a portion ofthe bridgecircuit of Fig. 5

showing certain voltage relations. I

, An example of application to the push-pull circuit is shown in Fig. 1in which only the tubes and the neutralizing branches are shown. Thisreproduction is carried out in the simplest manner in that a cold tubeis used in the neutralizing,

or a geometrically exact imitation with or with-.

out glass bulb.

It was found that especially in case of short waves, considerabledifiiculties are encountered 36 owing to thepassing of an electroncurrent be- H tween cathode and grid and between cathode and "anode.This electron current in fact, changes on -the one hand the capacitiesbetween cathode and x grid and between cathode and anode, owing to its 4:jspace charge, and on the otherhand, it produces a damping of thegrid-cathode path and a negative damping of the anode-cathode path..These conditions produce the following actions V uponneutralizatiomTheamplifier may be for example neutralized in that 'a high-frequencyalternating potential is applied to the input terminals while thepotential at the output is reduced to zero. If the tube is a non-heatedtube, and a neutralizing-condenser forms an exact 're- 5o production ofthe tube, the output potential is equal to zero, 1. e., the stage isneutralized. Now

if the tube is heated, so that a direct grid current flows,the outputpotential is no longer zero. However, neutralization can bereestablished by 56; increasing accordingly the grid-cathode capacity bythe neutralizing-condenser. At larger grid currents, a matching of thereactances is no longer sufficient, and it would be necessary to providecorresponding damping for the grid-cathode path of theneutralizing-condenser, in order to reproduce in the neutralizing-branchthe phase displacement produced by the grid current. This matching wouldhowever only be valid for a single operating condition.

Wholly equivalent conditions appear, if for carrying out theneutralization an alternating plate potential is applied to the outputterminals, and the alternating grid potential at the input issuppressed.

Hitherto these influences of the electron current to the grid have beenreduced in that sufliciently high capacities were connected in parallelto the electrode capacities so as to reduce the influence of theelectrode capacities. This method is useless as such in case ofultra-short waves, since on the one hand the capacities must bemaintained as low as possible, while on the other hand, the additionalparallel capacities can be connected only across a considerable linesection.

This invention relates to a means and method by which the detrimentalinfluences of the gridcathode path andanode-cathode path can beeliminated without prejudicing the operation especially at ultra-shortwaves. In accordance with the invention, the inductances of theelectrode lead-ins and the corresponding inductances of theneutralizing-branches are so chosen that a for the alternating gridpotential there are formedother equi-potential points besides the outputterminals (bridge diagonals) as well as all anodecathode paths and thecorresponding paths of the neutralizing branches. For the alternatingplate potential, besides the input terminals (other bridge diagonals)also all grid-cathode paths and. the corresponding paths of theneutralizingbranches are formed to effect equi-potential points, so thatvariations in these tube paths, cannot disturb the equilibrium of thebridge.

tube to the electrodes. Although the inductances of the lead-inssituated within the tube could as such be dimensioned in accordance withthe data according to the invention as discussed, froma practical pointof view, the correct conditions will haveto be assured by the connectionof inductances outside the glass bulb, since for reasons connected withthe manufacturing methods of the glass bulb, the electrode lead-ins willnot always have the dimensions herein required.

By the designation electrode lead-ins there is meant the connectionsinside and outside the to these paths.

While in the anode-neutralizing circuit solely the grid-cathode capacityand in the grid neutralizing circuit solely the anode-capacity can beformed into additional equi-potential points, both conditions can alsobe fulfilled at the same time in case of the push-pull circuit. For thisreason the full result can be obtained only when applying the inventionto the push-pull circuit.

It should also be pointed out that the change in the capacities by thespace charge is not entirely without influence upon the neutralization,however in applying the invention its influence will be considerablyreduced;

For a clearer explanation, the push-pull circuit according to Fig. l isredrawn in Fig. 2 in connection with the conditions of the bridge. Thecapacities are the electrode capacities assumed to be concentrated inthe center of the electrodes, and the corresponding capacities of theneutralizing-branches. ignated by letters such as LA, LG and LR: areassumed to be concentrated in the electrode leadins (not especiallyshown in Fig. 1). As in all lines (in contrast to coils), they are notdetermined by the lines proper, but by the entire closed circuit. Wheninterrupting the cathode connections of all tubes, and neutro-branches,and when short-circuiting all capacities, CAG the circuit starting atG1, Fig. 2, across A1, G2, A2 and back to G1 furnishes a resultantinductance, namely, 4:X(LA+LG). A closed circuit starting from A1 acrossthe anode of tube I, the short-circuited capacity CAK of tube I, point Kand anode a of the neutro-branch l and back, furnishes an inductance 2(LA+LK). Acorresponding closed circuit from G1 across K results in 2(LG+LK)- The three inductances LA, Lo and L1; thus are exactly defined.

In accordance with the invention, the induc-- tances LA, LG and LK andthe corresponding inductances of the neutro-branches are so chosen thatwhen applying a potential to a bridge diagonal, not only the otherbridge diagonals but also the electrode paths lying in the direction ofthese other diagonals will have zero voltage. When applying analternating grid potential to the points G1 and G2 an ordinaryneutralizing circuit provides zero voltage between A1, K and A2. Throughproper choice of the inductances according to the invention, thevoltages at all anode-cathode paths likewise become zero. The apparentresistance of the anode-cathode paths thus is of no, importance toneutralization. This will be readily understood, when considering thatvoltages (and hencealso resistances) are applied These alternatingpotentials have no effect at the terminals G1 and G2.

On the other hand, when applying an alterable subscript, alongside thearrows.

In Fig. 3, one of the two tube systems is shown again. When applying analternating grid potential Uebetween K and G1, the voltage between K andA1 is zero owing to the equilibrium of the bridge, so that the samealternating grid The inductances des- 1 potential UG lies between A1 andG1, and which for A1-G1 and K-G1 will be found as follows:

. CAK shall have zero voltage, so that therefore This value inserted inthe third equation gives,

Between G1-A1 there exists the same voltage 1 j 2 Jc(.l o wCAG Ccx willbe without voltage when: r

The two conditions furnish:

which gives the proper dimensioning for this example in accordance withthe invention. In pratctice these relations will be attained through tesThese relationships assume another form in case of tubes with concentricelectrode lead-ins for the purpose of reducing the lead-in induction andfor other reasons. pushgpull circuit with such tubes is shown in Fig.

L ZL IL C KZ AK: AG

On the one side of the tube, the lines leading to grid and cathode andon the other side, the lines leading to anode and cathode are broughtout concentrically arranged to each other. The

principal difference as compared with the case,

shown in Fig. 2, resides in that the concentric lead-ins producemagnetic coupling between the electrodes. concentric tubings is equal totheself induction of the outer tubing. Fig. 6 shows one of the two tubesystems of Fig. 5. In place of self inductions and capacities, thedesignations of the reactances are immediately indicated.

The bridge circuit of a- The counterinduction between two I 2,159,731When applying an alternating grid potential,

the points A1, K1 and K2 have the same potential owing to theequilibrium of the bridge, In Fig. 6,

CAK and Can, means connecting together the endsof the inductances LK ofthe tube networks remote from the cathodes and the corresponding Itfollows from the above that;

J3 Kl M1)( K2 XM2) By eliminating the currents and voltages there is:

The conditions for the zero voltage of the gridcathode'path atappliedalternating plate potential are obtained by interchanging theindices 2' and 1 and A with G.

the values Xe, XMl, XA, XK2 any two values thereof can be selected andtheir value so chosen that the two conditions are fulfilled thereby. In

this connection the lead-in inductances may also become negative, i. e.,they may become capacities.

Having described our invention, what we claim as novel and desire tosecure by Letters Patent is:

1. An amplifying stage comprising a pair of input terminals and a pairof output terminals, a pair of tube networks forming a push-pull circuiteach of said tubes including an anode, a cathode and a grid electrode,inherent capacity between thetube elements comprising CAe between saidanode and grid, CAK between said anode and cathode and CGK between saidgrid and cathode, said tube networks including leads for connecting saidtube elements in the circuit, inductance LK I in the lead from eachcathode, inductance LA in the lead from each anode and inductance LG inthe lead from eachgrid, said inductances being related to saidcapacities substantially in accordance with the proportionLA:LK.LG=CGK.CAG.'CAK

a pair of balancing networks composed of capacities and inductancessubstantially identical both as to arrangement and magnitude with saidtube network capacities and inductances,

whereby each of said balancing networks includes inductances Lx, LA andLa, and capacities CAG,

of said one tube network remote from the grid and the corresponding endof inductance LG of said other balancing network together and to aninput terminal and means for connecting the In view of the fact that thetube capacities reend of inductance LG remote from the grid of saidother tube network and the corresponding end of inductance LG of saidone balancing network together and to the other input terminal, wherebyvoltage impressed between the output terminals produces no difference ofpotential either between the input terminals or across CGK of eithertube and conversely voltage impressed between said input terminalsproduces no difference of potential either between the output terminalsor across CAK of either tube in spite of the existence of inherentinductance in said leads.

2. An amplifying stage comp-rising a pair of input terminals and a pairof output terminals, a pair of tube networks forming a push-pull circuiteach of said tubes including an anode, a cathode and a grid electrode,inherent capacity between the tube elements comprising CAG between saidanode and grid, CAK between said anode and cathode and Cox between saidgrid and cathode, said tube networks including leads for connecting saidtube elements in the circuit, inductance LK in the lead from eachcathode, inductance LA in the lead from each anode and inductance Le inthe lead from each grid, and wherein the following proportionissatisfied LK:LG=CAG:CAK, a pair of balancing networks composed ofcapacities and inductances substantially identical both as to theory andmagnitude with said tube network capacities and inductances, wherebyeach of said balancing networks includes inductances LK, LA and LG, andcapacities CAG, Cm and Cox,

means connecting together the ends of the inductances Lx of the tubenetworks remote from the cathodes and the corresponding ends of theinductances Lx of the balancing networks, means for connecting the endof the inductance LA remote from the anode of one tube network and thecorresponding end of inductance LA of one balancing network together andto one of said output terminals, means for connecting the end ofinductance LA remote from the anode of the other tube network and thecorresponding end of inductance LA of the other balancing networktogether and to the other output terminal, means for connecting the endof inductance L of said one tube network remote from the grid and thecorresponding end of inductance LG of said other balancing networktogether and to an input terminal and means for connecting the end ofinductance LG remote from the grid of said other tube network and thecorresponding end of inductance Le of said one balancing networktogether and to the other input terminal, whereby voltage impressedbetween the output terminals produces no difference of potential eitherbetween the input terminals or across CGK of either tube in spite of theexistence of inherent inductance in said leads.

3. An amplifying stage comprising a pair of input terminals and a pairof output terminals, a pair of tube networks forming a push-pull circuiteach of said tubes including an anode, a cathode and a grid electrode,inherent capacity between the tube elements comprising CAG between saidanode and grid, CAK between said anode and cathode and Cox between saidgrid and cathode, said tube networks including leads for connecting saidtube elements in the circuit, inductance L1; in the lead from eachcathode, in-

ductance LA in thelead from each anode and inductance LG in the leadfrom each grid, and wherein the following proportion is satisfiedLA:LK=CGK:CAG, a pair of balancing networks composed of capacities andinductances substan: tially identical both as to theory and magnitudewith said tube network capacities and inductances, whereby each of saidbalancing networks includes inductances LK, LA and LG, and capacitiesCAG, CAK and Cox, means connecting together the ends of the inductancesLK of the tube networks remote from the cathodes and the correspondingends of the inductances Lx of the balancing networks, means forconnecting the end of the inductance LA remote from the anode of onetube network and the corresponding end of inductance LA of one balancingnetwork together and to one of said output terminals, means forconnecting the end of inductance LA remote from the anode of the. othertube network and the corresponding end of inductance LA of the otherbalancing network together and to the other output terminal, means forconnecting the end of inductance LG of said one tube network remote fromthe grid and the corresponding end of inductance LG of said otherbalancing network together and to an input terminal and means forconnecting the end of inductance Lo remote from the grid of said othertube network and the corresponding end of inductance LG of said onebalancing network together and to the other input terminal, wherebyvoltageimpressed between said input terminals produces no difference ofpotential either between the output terminals or across CAK of eithertube in spite of the existence of inherent inductance in said leads.

WERNER BUSCHBECK. HANS PROST.

D l S C L A l M E R 2,159,731.Werner Buschbeck and Hans Prost, Berlin,Germany. NEUTRALIZING HIGH-FREQUENCY PUSH-PULL AMPLIFIERS. Patent datedMay 23, 1939. Disclaimer filed December 1, 1939, by the assignee,Telefunlcen Gesellschajt fair Dmhtlose Telegraphie, m. b. H. Herebyenters this disclaimer to the three claims in said patent.

[Ofiicial GazetteDecember 26, 1939.]

